In accordance with industry accepted best practices we ask that users limit their copy / paste of copyrighted material to the relevant portions of the article you wish to discuss and no more than 50% of the source material, provide a link back to the original article and provide your original comments / criticism in your post with the article.

Original Message

Zone runs from the northern most part of California into southern British Columbia, Canada. It has a history of rupturing along the entire subduction zone, resulting in massive M9+ events all along the Oregon and Washington coasts. The last such Cascadia megathrust earthquake was on January 26, 1700, at approximately 9pm, and was a M9+ event. It caused tsunamis in Japan and North America, and submerged & flooded coastal forests in Oregon and Washington. The Cascadia Subduction Zone is nearly a twin of the Sumatra Subduction Zone (which caused the devastating 2004 Indian Ocean tsunami, produced a measurable wobble of the Earth, and left a gravity scar in the earth.) A Cascadia megathrust earthquake could last up to five minutes. The subduction is causing the land between the coast and the Cascade mountains to bow upward; when a great quake hits, inland areas (like Portland) could lose as much as 6 feet elevation in less than five minutes, as the bow is released. During these Cascadia earthquakes, the whole fault is in motion; it isn't a pinpoint event like most crustal earthquakes; all western Oregon and Washington will experience terrific shaking. Aftershocks could themselves be major earthquakes, capable of substantial damage, further compounding the destruction; a large quake can produce hundreds of aftershocks, most of which occur in the weeks immediately following the event, often starting just a few hours after the initial event. Although the average between Cascadia earthquakes is 500 years, that number is misleading. Cascadia earthquakes come in. Clusters of typically three to five M9+ earthquakes, spaced approximately 300 years apart. A long quiescent period between clusters typically lasts from 500 to a little over a 1000 years. The 1700 earthquake was about the fourth event in its cluster; we're either overdue for the next one, or in a quiescent period. New evidence suggests the southern half of the Cascadia subduction zone also produces M8+ events even during quiescent periods between the M9+ clusters; these quakes will shake Portland, but Seattle not as much. If we include the smaller M8+ Oregon Cascadia earthquakes, the average between quakes shrinks to only 250 years. The duration of our current quiet period has already exceeded 75% of the quiet periods between quakes in Cascadia's history. Chris Goldfinger of OSU, who has studied Cascadia extensively, now says he expects either a M8+ or M9+ quake before 2060. A M9+ Cascadia earthquake will damage cities and towns between the Cascades and the coast, from Canada to California. t's doubtful that FEMA is prepared for such an event.

We can expect more damage and destruction in the Pacific Northwest from a large (total rupture) Cascadia megathrust earthquake than Japan experienced in their 2011 Sendai (Honshu) quake for two reasons: Cascadia routinely ruptures along a much longer fault, and our seismic building codes are significantly weaker than Japan's (our building codes only anticipate a M7 quake, whereas Japan's codes anticipate M9 quakes).